In recent years, with the inventions and marketing of optical amplifiers (EDFA: Erbium Doped Fiber Amplifier), a required OSNR (optical signal noise ratio) has been remarkably relaxed. With this situation, a super high speed optical transmission in which a transmission capacity of one wavelength exceeds 10 Gbit/s has been put into practical use, and globally introduced. Further, the costs are further reduced with the use of the broadband of the optical amplifier (that is, by introduction of linear repeaters that can amplify a given wavelength band together).
Among the linear repeaters, attention has been paid to an optical Raman amplifier (also called “DRA (distributed Raman amplifier)”). The optical Raman amplifier is an optical amplifier having a wavelength band close to 100 nm, and can amplify an optical signal of a broadband by a low noise. The optical Raman amplifier is disposed on a receiver side of a signal light with respect to an optical transmission line functioning as an amplification medium, and receives a pump light in a direction opposite to a propagation direction of the signal light. A stimulated Raman scattering effect is obtained by incidence of the pump light to amplify the signal light.
FIGS. 1-1A and 1-1B illustrate a configuration of an optical transmission system using an EDFA 102-1 and an EDFA 102-2 as optical amplifiers, and a transition of an optical power to a transmission length, respectively. FIGS. 1-2A and 1-2B illustrate a configuration of an optical transmission system using an EDFA 102-3 and an EDFA 102-4 as optical amplifiers and an optical Raman amplifier module 104, and a transition of an optical power to a transmission length, respectively.
A difference between both systems illustrated in FIGS. 1-1A and 1-2A resides in a power (gain) of the signal light input to repeaters 100-2 and 100-4 which are installed downstream of optical fiber transmission channels 101-1 and 101-2, respectively. In the system illustrated in FIG. 1-2, optical Raman amplification (Raman gain) occurs within the optical fiber transmission channel 101-2 with the aid of the pump light output from a pump light source 103. For that reason, it is found that the power of the signal light rises from the middle of the optical fiber transmission channel 101-2. As a result, the power of the signal light input to the EDFA 102-4 of the repeater 100-4 (FIG. 1-2) becomes larger than the power of the signal light input to the EDFA 102-2 of the repeater 100-2 (FIG. 1-1). Thus, the optical transmission system illustrated in FIG. 1-2A can reduce the deterioration of the OSNR to enable a long-distance optical transmission.
The advantage is obtained as described above. On the other hand, the installation of the optical Raman amplifier into the optical transmission system automatically incurs an increase in the optical power that is input to an optical fiber. For that reason, an optical power of 23 to 25 dBm (200 to 300 mW) at the maximum is input to the optical fiber in the case of a single wavelength pump, and an optical power of an integral multiple of that optical power is simply input to the optical fiber in the case of a wavelength multiplexing pump
From the viewpoint of the safety standards of laser, if light is confined in the optical fiber, safety is fundamentally kept. However, once light is leaked from the optical fiber due to attachment/detachment of a connector or disconnection of the optical fiber, energy is consumed, and the environment is adversely affected by the leaked light. For that reason, the optical Raman amplifier suffers from the following four problems.
A first problem resides in a damage of a connector end face. If a foreign matter is attached onto the connector end face, the end face is damaged by a high-output light. In particular, there has been known that phosphor bronze used in a related art FC connector is problematic, and in recent years, an FC connector using no phosphor bronze is mainstream.
A second problem resides in a phenomenon called “fiber fuse”. That is, there exists a phenomenon that a core of the optical fiber is partially melted by the light of the high output. There has been known that the melted part is transferred toward a light source while emitting the visual light. For that reason, the optical fiber after the melted part has been transferred is formed with a hollow line in a longitudinal direction of a core area. The transfer phenomenon is continued until the output of the light source is shut down or the optical power is powered down to a threshold value.
A third problem resides in human body safety (provision under JIS C 6802) against laser beam irradiation when the optical transmission line is disconnected or the connector is detached. At present, there is proposed a method in which an output of the light source is automatically stopped by application of a connector having a shutter function or detection of a reflected light.
A fourth problem resides in a damage of a fiber coating. When the optical transmission fiber is put under the high power environments due to the installation of the optical Raman amplifier, if an optical fiber 202 is folded or bent, there is a possibility that the fiber coating may be damaged. For example, as illustrated in FIG. 2, when the optical fiber 202 that connects the devices is deformed with an extremely small radius of curvature within an office building 200, there is a possibility that the above problem occurs. In this case, the light leaked from the optical fiber is absorbed by a coating material, and produces heat. In the worst case, the coating material may take fire.
Accordingly, in order to safely operate an optical communication system, it is very important to solve the above problems. For that reason, a guide line is prepared by, for example, ITU-T (International Telecommunication Union-Telecommunication).
The related art for solving the fourth problem among the above-mentioned problems has proposed a structure corresponding to claim 3 in Japanese Unexamined Patent Application Publication No. 2003-264509. FIG. 3 illustrates a system example corresponding to that structure. Referring to FIG. 3, a transmitter module 301 and a receiver module 302 are connected by an optical fiber transmission line 300.
The transmitter module 301 includes a light source 303 for detection of the fiber bending, an optical fiber 304, and a coupler 305. The coupler 305 is used for multiplexing a detection light output from the light source 303 for detection of fiber bending and an optical main signal of the optical fiber 304.
The receiver module 302 includes a pump laser 306 that outputs a pump light, an optical multiplexer 307 for multiplexing the pump light into an optical fiber transmission line 300, a coupler 308 for demultiplexing the detection light that has propagated in the optical fiber transmission line 300, an optical filter 309, an optical receiver 310 for detection of the fiber bending, and a pump laser controller 311 that controls the pump laser on the basis of measurement results of the optical receiver for detection of the fiber bending. The pump laser controller 311 and the pump laser 306 are connected by a control line 312.
In this example, a wavelength band of 1625 to 1675 nm which is large in bending loss is mainly used for a wavelength of the light source 303 for detection of fiber bending. The optical receiver 310 for detection of the fiber bending within the receiver module 302 receives the optical signal output from the light source 303 for detection of fiber bending, and analyzes data superimposed on the optical signal. When the optical receiver 310 for detection of the fiber bending cannot receive data, or when the optical intensity is lower than a given value, the optical receiver 310 for detection of fiber bending detects the occurrence of the fiber bending. With this detection, the pump laser controller 311 controls the output light intensity of the pump laser 306 to be powered down or shut down.